The problems posed by substances that deplete the atmospheric ozone layer were addressed at Montreal, where the protocol was signed imposing a reduction in the production and use of chlorofluorocarbons (CFC). This protocol underwent amendments which imposed the phasing out of CFCs and extended the regulation to other products, including hydrochlorofluorocarbons (HCFC).
The refrigeration and conditioned-air production industry has invested greatly in the replacement of these refrigerant fluids, and hydrofluorocarbons (HFC) have accordingly been marketed.
In the motor vehicle industry, the air-conditioning systems of vehicles marketed in many countries swapped from a chlorofluorocarbon refrigerant fluid (CFC-12) to a hydrofluorocarbon refrigerant fluid (1,1,1,2-tetrafluoroethane: HFC-134a), which is less harmful to the ozone layer. However, with regard to the objectives set by the Kyoto protocol, HFC-134a (GWP=1300) is considered as having a high warming potential. The contribution to the greenhouse effect of a fluid is quantified by the GWP (global warming potential), which summarizes the warming potential, taking a reference value of 1 for carbon dioxide.
The majority of refrigeration installations operate on the principle of the vapor compression cycle. According to this principle, a refrigerant fluid is evaporated at low pressure, by taking heat from a first surrounding medium. The vapor thus formed is then compressed by means of a compressor and then passed into a condenser in which it is converted into liquid form, giving rise to a release of heat in a second surrounding area. The liquid thus condensed then circulates in an expansion vessel, at the outlet of which it is converted into a two-phase mixture of liquid and vapor, which is finally introduced into the evaporator where the liquid is again evaporated at low pressure, which completes the cycle. In a supercritical cycle, there is no condensation, and the condenser is referred to as a cooler.
Regulating the expansion vessel allows control of the overheating at the compressor inlet and optimizes the functioning of the installation.
The yield of a compression system depends on the components, the architecture of the system, the operating conditions and the fluid. The fluids used may be pure substances or azeotropic or zeotropic mixtures. The temperature glide of a fluid is defined as being the difference in temperature between the bubble point and the dew point at constant pressure. In compression systems with a supercritical cycle, the temperature glide is considered as being the difference in temperature between the inlet and outlet of the cooler.
In a theoretical compression cycle, the phase changes (condensation/evaporation) are at constant pressure. With one-component fluids and azeotropic mixtures, the condensation and evaporation are at constant temperature. The temperature glide is then zero.
Air-conditioning units, refrigeration machines and heat-exchange pumps operate on the same principles.
Since carbon dioxide is nontoxic, nonflammable and has a very low GWP, it has been proposed as a refrigerant for air-conditioning systems as a replacement for HFC-134a. However, the use of carbon dioxide has many drawbacks, especially associated with the very high pressure of its use as a refrigerant fluid in existing devices and techniques.
The use of carbon dioxide alone in a supercritical cycle or in combination with other compounds, such as HECs, in a refrigeration system or a common heat-exchange pump may thus lead to unacceptable reductions in energy efficiency if the surrounding medium (air, water or glycol-water) does not have a temperature glide as high as that of the fluid used. The consequence is that the difference between the mean temperature of the phase change of a mixture comprising carbon dioxide and the mean temperature of the surrounding medium increases, thus resulting in an increase in the difference between the evaporation and cooling pressures, which has the direct consequence of reducing the energy efficiency of the refrigeration system or of the heat-exchange pump. This energy efficiency also decreases following the degradation of efficiency of the exchangers as a result of the variation of the difference in temperatures between the refrigerant fluid and the surrounding medium across the exchanger.
Document U.S. Pat. No. 6,073,454 discloses the use of a co-fluid in combination with carbon dioxide as a refrigerant for reducing the working pressure. However, the refrigeration performance is insufficient.